Method of treating nanoparticles using a proton exchange membrane and liquid electrolyte cell

ABSTRACT

One embodiment of the invention includes an electrochemical cell including a proton exchange membrane and a method of treating nanoparticles using the same.

TECHNICAL FIELD

The field to which the disclosure generally relates includes methods oftreating nanoparticles.

BACKGROUND

The electrochemical treatment of large quantities of nanoparticles,including coating, stripping, oxidation, reduction, cleaning, dealloyingof nanoparticles and so on, has long been a technical barrier for moreextensive applications of this technique in many fields such as for fuelcells, batteries, and heterocatalysis. Heretofore, such electrochemicaltreatment has resulted in non-uniform treatment of the nanoparticles.

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION

One embodiment of the invention includes a method of using anelectrochemical cell including a liquid electrolyte, a working electrodewith nanoparticles supported thereon, a counter electrode, and a polymerelectrolyte membrane completely separating the liquid electrolyte at theworking electrode side and liquid electrolyte at the counter electrodeside.

Other exemplary embodiments of the invention will become apparent fromthe detailed description provided hereinafter. It should be understoodthat the detailed description and specific examples, while disclosingexemplary embodiments of the invention, are intended for purposes ofillustration only and are not intended to limit the scope of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will become more fully understoodfrom the detailed description and the accompanying drawings, wherein:

FIG. 1 illustrates an electrode chemical cell according to oneembodiment of the invention.

FIG. 2 is a graph showing a comparison of the platinum supported ongraphitized carbon pre-oxidation curve at 1.2V(RHE) obtained by thiscell design versus by a conventional electrochemical cell.

FIG. 3 is a graph showing a comparison of fuel cell performance data formembrane electrode assemblies (MEAs) containing the 1.4V-pretreated Pton graphitized carbon as cathode catalyst and for MEAs of non-treated Pton graphitized carbon catalyst.

FIG. 4 illustrates a multi-cell according to one embodiment of theinvention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description of the embodiment(s) is merely exemplary innature and is in no way intended to limit the invention, itsapplication, or uses.

FIG. 1 illustrates an electrochemical cell 10 according to oneembodiment of the invention. The electrochemical cell 10 may include acontainer 12 that holds a liquid electrolyte 14. The liquid electrolyte14 may be an aqueous acid solution, for example including perchloricacid, sulfuric acid, or phosphoric acid. The liquid electrolyte may alsobe any salt solution, like copper sulfate, lead sulfate, copper nitrate;or combination of salt and acid solutions. The container 12 may be madefrom any of a variety of materials, for example PTFE, glass, or otheracid resistant material. The electrochemical cell 10 may include aworking electrode 16 and a counter electrode 22. Suitable material forthe working electrode 16 and counter electrode 22 include, but are notlimited to, metals such as Pt, Au, or graphite. The working electrode 16and the counter electrode 22 may be in the form of gauze. The gauzematerial serves the function of increasing the contact area anddecreasing the mass transport resistance. The electrochemical cell 10may include nanoparticles 20 to be treated, which may be spread on asupport material 18 such as a first carbon cloth. The nanoparticles 20are electrically conductive and may be solid particle, shells withhollow cores, or strands of connected particles. For example, thenanoparticles 20 may include, but are not limited to carbon, Pt or Ptalloy, Ni or other metals, TiO₂, or electrically conductive shells. Thefunction of the first carbon cloth 18 is increasing the contact areabetween the nanoparticles and the supporting material. The supportmaterial or first carbon cloth 18 is further supported by the workingelectrode 16 which may be a gauze material including, for example,platinum, gold, or graphite. A second platinum gauze and a second carboncloth 24 are used as the counter electrode. The counter electrode maycontain a layer of Pt/C nanoparticles or Pt black spread on the carboncloth. The function of these Pt/C nanoparticles is to increase theactive surface area of the counter electrode 22. Depending on theelectrochemical reaction occurring on the counter electrode 22, thematerial of counter electrode may also be Cu, Pb, Ag, or other metals ormetal alloys. The arrangement of the gauze working electrode 16,overlying first carbon cloth 18, and the gauze counter electrode 22 andunderlying second carbon cloth 24 minimizes the in-cell electronicresistance which can cause non-uniform potential distribution in theworking electrode 16 and counter electrode 22. The electronic resistancein the thickness direction is very small.

A polymer electrolyte membrane 26 is interposed between the supportmaterial 18 and the counter electrode 22 so that the polymer electrolytemembrane serves to separate a working electrode compartment 7 and acounter electrode compartment 9 of the cell 10 wherein the polymerelectrolyte membrane 26 completely separates the liquid electrolyte 14in the working electrode compartment 7 from the liquid electrolyte 14 inthe counter electrode compartment 9 of the cell 10. The second carboncloth 24 may be interposed between the counter electrode 22 and thepolymer electrolyte membrane 26. The function of the second carbon cloth24 is to reduce the stress that the Pt gauze applies on the membrane.The second carbon cloth may also function as a support for Pt/Cnanoparticles, in case a layer of Pt/C nanoparticles or Pt black isincluded as a part of the counter electrode 22.

In one embodiment of the invention, the working electrode 16, firstcarbon cloth 18, nanoparticles 20, membrane 26, and optionally thesecond carbon cloth 24 and counter electrode 22 are all supported by thecontainer 12. This prevent damage to materials such as the membrane 26.

A reference electrode 28 may be provided immersed in the liquidelectrolyte 14 on the working electrode side of the cell 10. Suitablereference electrodes 28 include, but are not limited to a Ag/AgClelectrode, a Calomel electrode, or a reversible hydrogen electrode. Agas purge tube 30 may be provided immersed in the liquid electrolyte 14in the working electrode compartment 7 of the cell 10. A cover 32 may beplaced over the container 12 with a seal or gasket 34 interposed betweenthe cover 32 and the container 12. Both the cover 32 and the container12 may be made from a material including, but not limited to,polytetrafluororethylene, glass, or other acid-resistant material. Apotential is applied across the electrodes to treat the nanoparticles20, using an energy source such as a battery. This arrangement may beutilized for coating, stripping, oxidation, reduction, cleaning, ordealloying the nanoparticles 20.

This design ensures uniform potential and uniform current densitydistribution throughout the working electrode 16 and counter electrode22 even at high current conditions and consequently ensures a uniformand highly efficient electrochemical treatment of the nanoparticles. Thecell design combines some advantages of the polymer electrolyte membranefuel cell and some of the conventional liquid electrolyteelectrochemical cell. In the case where the electrochemical reaction atthe counter electrode 22 is not the reverse reaction of the workingelectrode 16 (for example when H₂ or O₂ evolution occurs at the counterelectrode), the design can easily prevent the reaction products (H₂ orO₂) from diffusing into the working electrode 16. As the nanoparticles20 are immersed in the liquid electrolyte 14, the utilization of thenanoparticles 20 approaches 100%, i.e. all of the nanoparticles 20 canbe treated and can be easily washed out after the treatment. Neither ofthese features can be achieved for the catalyst layer in a polymerelectrolyte membrane fuel cell, in which the catalyst layer is mixedwith a solid ionomer phase.

As an example, FIG. 2 shows a comparison of the platinum supported ongraphitized carbon (Pt/GrC) pre-oxidation current at 1.2V(RHE) by usingan electrochemical cell according to the present invention versus thesame process in a conventional electrochemical cell. The much highercurrent for the conventional cell is ascribed to the oxidation of H₂diffusing from the counter electrode, which is not a desirable processand prevents monitoring the progress of the desired treatment of thenanoparticles through a simple current measurement. The actual Pt/GrCpre-oxidation current is achieved with the electrochemical cellaccording to one embodiment of the invention, with the current droppingdown to less than 10 mA/g(Pt/GrC) in the initial 10 minutes. As such,the electrochemical cell shown in FIG. 1 can be utilized toelectrochemically treat large quantities of nanoparticles withuniformity, high efficiency, and facile monitoring of the state ofprogress of the treatment.

As an example of an application of this cell, FIG. 3 shows thatpre-oxidized Pt nanoparticles supported on graphitized carbon by usingthe present invention give higher fuel cell performance than non-treatedPt nanoparticles supported on graphitized carbon. FIG. 3 shows acomparison of fuel cell performance data at the conditions indicated inthe graph for various membrane electrode assemblies (MEAs), which refersto the combination of the anode catalyst, cathode catalyst, and themembrane. The solid curves are for MEAs containing the 1.4V-pretreatedPt on graphitized carbon as cathode catalyst. The dashed curves are forMEAs of non-treated Pt on graphitized carbon catalyst. At 1.5 A/cm², theimprovement is 25 mV. At 0.6 A/cm², the improvement is as much as 50 mV.In one embodiment, the nanoparticles 20 used in a H₂/air proton exchangemembrane (PEM) fuel cell operated at high current densities can achievehigher voltage.

In various embodiments, the polymer electrolyte membrane 26 may includea variety of different types of membranes. The polymer electrolytemembrane 26 useful in various embodiments of the invention may be anion-conductive material. Examples of suitable membranes are disclosed inU.S. Pat. Nos. 4,272,353 and 3,134,689, and in the Journal of PowerSources, Volume 28 (1990), pages 367-387. Such membranes are also knownas ion exchange resin membranes. The resins include ionic groups intheir polymeric structure; one ionic component for which is fixed orretained by the polymeric matrix and at least one other ionic componentbeing a mobile replaceable ion electrostatically associated with thefixed component. The ability of the mobile ion to be replaced underappropriate conditions with other ions imparts ion exchangecharacteristics to these materials.

The ion exchange resins can be prepared by polymerizing a mixture ofingredients, one of which contains an ionic constituent. One broad classof cationic exchange, proton conductive resins is the so-called sulfonicacid cationic exchange resin. In the sulfonic acid membranes, thecationic exchange groups are sulfonic acid groups which are attached tothe polymer backbone.

The formation of these ion exchange resins into membranes or chutes iswell-known to those skilled in the art. The preferred type isperfluorinated sulfonic acid polymer electrolyte in which the entiremembrane structure has ionic exchange characteristics. These membranesare commercially available, and a typical example of a commercialsulfonic perfluorocarbon proton conductive membrane is sold by E. I.DuPont D Nemours & Company under the trade designation NAFION. Othersuch membranes are available from Asahi Glass and Asahi ChemicalCompany.

The use of other types of membranes, such as, but not limited to,perfluorinated cation-exchange membranes, hydrocarbon basedcation-exchange membranes as well as anion-exchange membranes are alsowithin the scope of the invention.

The electrochemical cell 10 may be used to coat nanoparticles 20 with acatalyst such as platinum to provide a plurality of supported catalystparticles. The supported catalyst particles may be combined with anionomer which may be the same as the material for the above describedmembrane material. The supported catalyst particles and ionomer may beapplied to both faces of a polymer electrolyte membrane of a fuel cell.The supported catalyst particles and ionomer may alternatively beapplied to a fuel cell gas diffusion media layer or onto a decal backingfor later application as desired.

The above description is for a single cell design. Another embodiment ofthe invention includes a multi-cell design or electrochemical multi-cell38. A schematic drawing of one embodiment is shown in FIG. 4, wherein40, 44, 46, 50, 52, and 56 are working electrodes similar to the workingelectrode 16 described above. The working electrodes 40, 44, 46, 50, 52,and 56 contain nanoparticles 20 to be treated, supported on Pt or Augauze or on other highly electronically conductive and acid-resistantmaterials. These working electrodes may be supported or sandwiched bybacking material. The appropriate types of backing materials include butare not limited to perforated PTFE board. The multi-cell design 38 alsoincludes counter electrodes 42, 48, and 54. Depending on theelectrochemical reaction occurring on the counter electrode, thematerial of counter electrodes 42, 48, and 54 may include Pt, Cu, Pb,Ag, or other metals or metal alloys. An electrolyte 60 fills each ofworking electrode compartment 64 and counter electrode compartment 66.Membranes 62 separate the electrolyte in the working electrodecompartments 64 from that in the counter electrode compartments 66. Themulti-cell design 38 may include a container 58, which may be glass,PTFE or other acid-resistant material. In one embodiment, the multi-cellmay have a cover made of acid resistant material (not shown). Gas may bepurged into each compartment. A reference electrode (not shown) may beplaced close to any of the working electrodes. One counter electrode maybe shared by multiple working electrodes.

When the terms “over”, “overlying”, “overlies” or “under”, underlying”or “underlies” or the like are used herein with respect to the relativeposition of layers or components to each other such shall mean that thelayers or components are in direct contact with each other or thatanother layer, layers, component or components may be interposed betweenthe layers components.

The above description of embodiments of the invention is merelyexemplary in nature and, thus, variations thereof are not to be regardedas a departure from the spirit and scope of the invention.

1. A method comprising: supporting electrically conductive nanoparticlesto be electrochemically treated over a working electrode, immersing theworking electrode with the supported nanoparticles in a liquidelectrolyte solution, immersing a counter electrode in the electrolytesolution, and immersing a polymer electrolyte membrane in theelectrolyte solution between the working electrode with nanoparticlessupported thereon and the counter electrode to define a workingelectrode compartment and a counter electrode compartment of the cell,applying a potential or a current across the electrodes to treat thenanoparticles.
 2. A method as set forth in claim 1 further comprisingimmersing a reference electrode on the working electrode side.
 3. Amethod as set forth in claim 2 further comprising providing a gas purgetube in the liquid electrolyte on the working electrode side of thecell.
 4. A method as set forth in claim 3 further comprising providing acontainer for holding the electrolyte solution and a cover over thecontainer.
 5. A method as set forth in claim 1 wherein the workingelectrode comprises a first carbon cloth and a gauze comprising a metalsupporting the carbon cloth, and wherein the particles are spread on thefirst carbon cloth.
 6. A method as set forth in claim 5 wherein thegauze comprises platinum or gold or graphite.
 7. A method as set forthin claim 5 wherein the counter electrode comprises a second carbon clothand a gauze material comprising a metal supporting the carbon cloth. 8.A method as set forth as claim 5 wherein the counter electrode comprisesa second carbon cloth supported by a gauze comprising platinum orsupported Pt nanoparticles.
 9. An electrochemical cell comprising: acontainer and a liquid electrolyte received in the container; a workingelectrode, and nanoparticles supported by the working electrode; acounter electrode; and a polymer electrolyte membrane separating liquidelectrolyte on the counter electrode side from liquid electrolyte on theworking electrode side of the cell.
 10. An electrochemical cell as setforth in claim 9 wherein the nanoparticles comprise at least one of Pt,Pt alloy, Ni, or other noble metals or metal alloys.
 11. Anelectrochemical cell as set forth in claim 9 wherein the workingelectrode comprises a first carbon cloth supporting the nanoparticles,and a gauze comprising a metal supporting the carbon cloth.
 12. Anelectrochemical cell as set forth in claim 11 wherein the gauzecomprises at least one of platinum or gold or graphite.
 13. Anelectrochemical cell as set forth in claim 9 wherein the counterelectrode comprises a second carbon cloth supported by a gauzecomprising a metal.
 14. An electrochemical cell as set forth in claim 13wherein the gauze comprises at least one of platinum or gold orgraphite.
 15. An electrochemical cell as set forth in claim 9 furthercomprising a reference electrode immersed in the liquid electrolyte onthe working electrode side of the cell.
 16. An electrochemical cell asset forth in claim 9 further comprising a gas purge tube immersed in theliquid electrolyte on the working electrode side of the cell.
 17. Anelectrochemical cell as set forth in claim 16 further comprising a coverover the container.
 18. A method as set forth in claim 1 wherein thenanoparticles comprise Pt/C, and the electrochemical treatment of thenanoparticles comprises an electrochemical oxidation step.
 19. A methodas set forth in claim 18 further comprising using the nanoparticles in aH²/air proton exchange membrane (PEM) fuel cell operated at high currentdensities to achieve higher voltage.
 20. An electrochemical multi-cellcomprising: a container and a liquid electrolyte received in thecontainer; at least two working electrodes, and nanoparticles supportedby the working electrodes; at least one counter electrode; and at leasttwo polymer electrolyte membranes separating liquid electrolyte incounter electrode compartments from liquid electrolyte in workingelectrode compartments in the multi-cell.
 21. An electrochemicalmulti-cell as set forth in claim 20 wherein the nanoparticles compriseat least one of Pt, Pt alloy, Ni, or other noble metals or metal alloys.22. An electrochemical multi-cell as set forth in claim 20 wherein theworking electrodes comprise a first carbon cloth supporting thenanoparticles, and a gauze comprising a metal supporting the carboncloth.
 23. An electrochemical multi-cell as set forth in claim 22wherein the gauze comprises at least one of platinum or gold orgraphite.
 24. An electrochemical multi-cell as set forth in claim 20wherein the counter electrodes comprise a second carbon cloth supportedby a gauze comprising a metal.
 25. An electrochemical multi-cell as setforth in claim 24 wherein the gauze comprises at least one of platinumor gold or graphite.